Characterization and evaluation of an integrated quality monitoring system for online quality assurance of external beam radiation therapy

Abstract Purpose The aim of this work was to comprehensively evaluate a new large field ion chamber transmission detector, Integral Quality Monitor (IQM), for online external photon beam verification and quality assurance. The device is designed to be mounted on the linac accessory tray to measure and verify photon energy, field shape, gantry position, and fluence before and during patient treatment. Methods Our institution evaluated the newly developed ion chamber's effect on photon beam fluence, response to dose, detection of photon fluence modification, and the accuracy of the integrated barometer, thermometer, and inclinometer. The detection of photon fluence modifications was performed by measuring 6 MV with fields of 10 cm × 10 cm and 1 cm × 1 cm “correct” beam, and then altering the beam modifiers to simulate minor and major delivery deviations. The type and magnitude of the deviations selected for evaluation were based on the specifications for photon output and MLC position reported in AAPM Task Group Report 142. Additionally, the change in ion chamber signal caused by a simulated IMRT delivery error is evaluated. Results The device attenuated 6 MV, 10 MV, and 15 MV photon beams by 5.43 ± 0.02%, 4.60 ± 0.02%, and 4.21 ± 0.03%, respectively. Photon beam profiles were altered with the IQM by < 1.5% in the nonpenumbra regions of the beams. The photon beam profile for a 1 cm × 1 cm2 fields were unchanged by the presence of the device. The large area ion chamber measurements were reproducible on the same day with a 0.14% standard deviation and stable over 4 weeks with a 0.47% SD. The ion chamber's dose–response was linear (R2 = 0.99999). The integrated thermometer agreed to a calibrated thermometer to within 1.0 ± 0.7°C. The integrated barometer agreed to a mercury barometer to within 2.3 ± 0.4 mmHg. The integrated inclinometer gantry angle measurement agreed with the spirit level at 0 and 180 degrees within 0.03 ± 0.01 degrees and 0.27 ± 0.03 at 90 and 270 degrees. For the collimator angle measurement, the IQM inclinometer agreed with a plum‐bob within 0.3 ± 0.2 degrees. The simulated IMRT error increased the ion chamber signal by a factor of 11–238 times the baseline measurement for each segment. Conclusions The device signal was dependent on variations in MU delivered, field position, single MLC leaf position, and nominal photon energy for both the 1 cm × 1 cm and 10 cm × 10 cm fields. This detector has demonstrated utility repeated photon beam measurement, including in IMRT and small field applications.


Conclusions:
The device signal was dependent on variations in MU delivered, field position, single MLC leaf position, and nominal photon energy for both the 1 cm 9 1 cm and 10 cm 9 10 cm fields. This detector has demonstrated utility repeated photon beam measurement, including in IMRT and small field applications. image-guided radiation therapy (IGRT), 3,4 volumetric-modulated arc therapy (VMAT), 5,6 and small field treatments utilized in SRT and SBRT are examples of radiation delivery practice with more intricate work flow than "conventional" radiation therapy. Due to this increased complexity, new quality assessment (QA) strategies have been developed, including patient-specific dose verification. [7][8][9][10] This patient-specific QA measurement is only performed once for a treatment course that can include as many as 44 sessions. During the course of treatment, errors may be introduced by changes in software, hardware, or human procedure. One strategy to address these potential treatment errors is online monitoring of every radiation therapy session. This goal can be accomplished by placing a transmission detector on the head of a linac and making dosimetric measurements of the radiation beams as they are being delivered to the patient. This online monitoring has potential to detect many potential treatment errors. 11 The value and importance of performing a measurement of the radiation delivered for each fraction of a course of external beam radiation therapy has been previously discussed by Mijnheer et al. 12 In fact, a dosimetry measurement is required by the national recommendations of Sweden and France, and is recommended by Royal College of Radiologists of the United Kingdom. 12 In this context, transmission detector systems will likely become a more prevalent quality assurance measure in the future.
Online photon beam dose verification with a transmission detector system has been previously demonstrated. Paliwal et al. 13 used a large area transparent transmission chamber mounted on the shielding tray that detected deviations from the initial treatment in photon beam fluence in subsequent sessions. Another strategy is the use of a flat, multiwire transmission-type ionization chamber, attached to the accessory holder of a linac. 14 One such system is known as the DAVID system (PTW-Freiburg, Freiburg, Germany) and has evaluated for the online detection of MLC discrepancies in IMRT deliveries. 15 COMPASS â (IBA Dosimetry, Schwarzenbruck, Germany) is a transmission detector consisting of 1600 plane-parallel ionization chambers. 16 It has been used for the online measurement of IMRT treatments and validated by Monte Carlo simulation. 17 18 as well as verification of adapted treatment fields. 19 Another transmission detector, named the "magic plate," has been developed using a 2D array of silicon diodes. 20,21 The VANILLA system uses monolithic active pixel sensors to measure ionizing radiation beam profiles. 22 Another monitor has been developed that utilized optical attenuation-based detectors to measure light produced in long scintillating fibers by the photon fluence at the linac head. 23 This work characterized the Integral Quality Monitor (IQM), developed by iRT Systems GmbH (Koblenz, Germany). The design of this commercially available device is based on the research prototype developed by Islam et al. 18 The aim of this work was to evaluate the IQM's effect on photon beam fluence, response to dose, detection of photon fluence modification, and the accuracy of the integrated barometer, thermometer, and inclinometer. Additionally, this research evaluates the dependence of the ion chamber's signal on MLC and photon beam characteristics selected based on the quality assurance recommendations of AAPM Task Group 142. 24 This publication represents original research, different from the previously published work of Islam et al. in that: 1) The earlier publication was for prototype device with a ion chamber and electronics design that was never commercially available, 2) This work utilized a new commercially available design that operates as a bluetooth wireless device, 3) the integrated inclinometer, barometer, and thermometer is evaluated, 4) the effect on photon beam percent depth dose (PDD) and profile is evaluated for energies beyond 6 MV, including 10 and 15 MV, 5) the scenario of the device losing power mid-treatment is evaluated, 6) the effect on photon beam profile for a IMRT sized field (1 9 1 cm 2 ) is characterized, 7) a dose-rate dependence of the ion chamber response is evaluated and addressed, 8) the change in ion chamber signal caused by a simulated IMRT delivery error is evaluated.

| MATERIAL AN D METHOD
The IQM is a commercially available quality monitoring system composed of a large area (26 cm 9 26 cm) position-sensitive ion chamber, barometer, thermometer, and inclinometer. The device attaches to the accessory tray holder of a linear accelerator, as shown in The creation of a checksum is an important aspect of the IQM's function. The digitized current produced in the IQM's ion chamber is recorded for every beam, control point, or segment during radiation delivery. At the end of a treatment session, each measurement, as well as the total signal, or checksum, can be compared to previous measurements. This baseline measurement could be performed during a patient-specific quality assurance measurement, before delivery of the first fraction. This allows for outlying deliveries to be quickly detected, possible even during treatment delivery.

2.A | Integrated quality monitoring system evaluation
To validate the large area detector's ability to make useful EBRT quality assurance measurements, the sensitive area of the detector should be sufficiently sized to intersect all linear accelerator beams. The linear accelerator used for this work was an Elekta Synergy (Stockholm, Sweden) with an Agility MLC to produce photon beams with nominal  Increased signal with increasing field size was interpreted as adequate coverage of the jaw aperture by the ion chamber active area.
No increase was interpreted as inadequate coverage.
To correct for temperature and pressure effects of the ion cham- To evaluate the large area detector's signal linearity, dose-rate dependence, reproducibility, and stability, a 6 MV photon 10 cm 9 10 cm field was delivered through the center of the active area while modulating total MU and dose rate. A measurement of 100 MU was repeated 10 times to evaluate reproducibility. The same measurement was repeated 9 times over 4 weeks to evaluate stability.

2.B | Effect of the monitoring system on the treatment beam
The attenuating effect of the monitoring system on photon beams was measured by comparing the charge produced in a Farmer type

2.C | Photon beam error detection
The useful application of the IQM for online photon beam quality assurance requires that it must be able to detect clinically relevant errors in photon beam delivery. By modifying a simple photon field, the magnitude of signal produced in the ion chamber can be changed. In this work, these modifications are used to simulate treatment delivery errors. A 10 cm 9 10 cm, representing a moderately sized normal field, and a 1 cm 9 1 cm, representing a small-sized field, baseline stability (1%), the magnitude of the modification was incrementally increased until the change in IQM signal was at least twice the standard deviation and the magnitude of detectable modification was reported.

2.D | IMRT and VMAT reproducibility and simulated IMRT delivery error
As a baseline, the reproducibility of IMRT and VMAT measurements was characterized with triplicate measurements of a pharyngeal tonsil plan for IMRT and prostate plan for VMAT. Then, a well-documented IMRT delivery error was selected to be simulated. 25 In the selected case, a patient with tongue cancer was  T A B L E 1 The percentage change of IQM signal when the baseline 6 MV photon beam, 10 9 10 cm 2 field and 100 MU, is changed with the listed modifications. For modifications that result in less than a 1% signal change, the magnitude of modification to give 1% signal change is recorded.  Fig. 4.
The dosimetric effects of the incorrect utilization of the device have not been investigated in this project. If the device was in place and not accounted for or accounted for while not in place, it would affect PTV dose in a way that is not characterized in this project.

3.C | Photon beam error detection
The three baseline measurements of the normal (10 cm 9 10 cm) field had similar reproducibility (0.15%) to the previously described reproducibility evaluation (0.14%), while the small (1 cm 9 1 cm) field had 0.5% reproducibility. The percentage change measured from the baseline of each modification is listed in Tables 1 and 2. IQM signal changes equal or greater than 1% (more than twice the standard deviation of the stability of the measurement) were consid- The IQM is less sensitive to single MLC leaf changes in the 10 cm 9 10 cm field than for the 1 cm 9 1 cm field.

3.D | Simulated IMRT delivery error
Repeated measurements of the pharyngeal tonsil IMRT plan resulted in a 0.15% standard in the checksum of each beam. Similarly, the repeated measurement of the prostate VMAT plan resulted in 0.16% standard deviation of the checksum for each arc. When the simulated IMRT error was delivered, the ion chamber signal of each segment increased by a factor of 11-238 times the baseline measurement, as shown in Table 3.

| CONCLUSION
Our investigation has demonstrated that the IQM is stable for online delivery quality assurance measurements. This device has been vali- Future work will evaluate the reproducibility of checksum measurements for 3D conventional, IMRT, and VMAT plans, with a range of patient target volume sizes that covers the range of clinically relevant photon treatments.

ACKNOWLED GMENT
We acknowledge iRT Systems GmbH for use of the IQM device, software, and assistance. The authors also further acknowledge the support of Richard Valicenti and the University of California, Davis Department of Radiation Oncology, where this work was performed.
T A B L E 3 The IQM signal of each segment for a representative beam of a pharyngeal tonsil IMRT plan. The plan was delivered correctly with the planed modulation and with a simulated error, where each segment of the plan was delivered without modulation and the MLC leaves open.